Delirium has been recognized and described since antiquity. It is a brain disturbance manifested by a syndrome of diverse neuro­­psychiatric symptoms. Various terms have been used for delirium, such as acute brain disorder, metabolic enceph­alopathy, organic brain syndrome, and ICU psychosis. The DSM-IV model views delirium as an acute reversible neuropsychiatric syndrome caused by general medical conditions and/or exogenous substances.1 DSM-IV criteria for delirium require a disturbance of consciousness or atten­tion and a change in cognition that de­velops acutely and tends to fluctuate in severity. Lipowski2 characterized de­lir­ium as a disorder of attention, wakefulness, cognition, and motor behavior.

Here we discuss the various subtypes (hypoactive, hyperactive, and mixed) of delirium and review the cases of 16 patients who met criteria for concurrent delirium and catatonia. These cases support the concept of a catatonic subtype of delirium. Our findings may have treatment implications that are yet to be determined.

Etiology, features, and subtypes Studies of noncognitive features of delirium, such as motor activity, have led to the conceptualization of hypo­active, hyperactive, and mixed subtypes based on the salient activity pattern.3 Numerous studies over the past 20 years have attempted to define these motor subtypes of delirium. These studies show various rates of the hypoactive, mixed, and hyperactive forms using differing criteria and definitions. Some studies have found that these subtypes predict etiology, clinical course, morbidity, presence of psychosis, and other factors.

Other studies of delirium have shown evidence for neurochemical differences according to motor subtype. For example, in a study of delirium tremens, elevated cerebrospinal fluid concentrations of homovanillic acid (HVA, a metabolite of dopamine) correlated with the degree of agitation.4 A more recent study examined urinary excretion of 6-sulfatoxymelatonin (6-SMT, a metabolite of melato­nin) in patients with hypoactive delir­ium.5 Levels of 6-SMT were markedly elevated in patients with hypoactive delirium, and were reduced during recovery.

Patients with the hyperactive subtype of delirium showed an opposite pattern, with initially low levels that increased with recovery.

In addition to etiology and clinical features, the motor subtype of delirium may affect treatment response. This view is supported by an open pro­spective study of delirium treatment in 79 patients with cancer.6 The study used rating scales to monitor severity of delirium after treatment with olanzapine for 7 days. Patients with the hypoactive form of delirium showed a significantly reduced response rate (48%) compared with those who had the hyperactive subtype (83%). However, an earlier smaller study with 24 patients found no difference in the efficacy of haloperidol and chlorpromazine for delirium according to motor subtype.7 A recent review supports the concept that motor subtypes in delirium may differ and that subtyping may prove useful for clinical and research goals.8